1. Field of the invention
This invention relates to a distributed feedback semiconductor laser device having a distributed reflector in a laminated crystal structure.
2. Description of the prior art
A distributed feedback semiconductor laser device, in which the refractive index or the absorption coefficient is changed at the same periodicity as the wavelength of the laser beam in the direction of the propagation of the laser beam, can easily achieve laser oscillation in a single longitudinal mode, and does not discontinuously change its oscillating wavelength even when the temperature or the excitation current level is changed rapidly. Accordingly, such a device is very useful as a light source of an optical communication system or optical measuring system.
In such a distributed feedback semiconductor laser device, it is critically important to prevent laser oscillation in a Fabry-Perot resonator mode. In other words, such a device must be constructed so that the cleavage facets of both the ends do not constitute a Fabry-Perot resonator. It has been proposed to provide such a construction by processing physically or chemically one or both of the ends of the device to be corrugated, or by forming one or both of the ends to be slanted so as not to be perpendicular to the growth surface of the laminated crystal structure (for example, OQE83-100, Vol. 83, No. 251, p57-64, Jan. 25, 1984, and Japanese Laid Open Patent Publication No. 52-86786).
In a semiconductor laser device having such a construction, however, the laser beam is scattered or reflected into a direction other than the light emitting direction at the facets to prevent oscillation in a Fabry-Perot resonator mode, thereby causing a significant loss, lowering the gain of the device. Hence, a distributed feedback semiconductor laser generally has a lower differential efficiency than that of a conventional semiconductor laser having a Fabry-Perot resonator.
In order to eliminate such a loss caused at the facets, it has been proposed that one of the facets be coated by dielectric layers so as to provide a lower reflectivity and the other facet be coated so as to provide a higher reflectivity. However, this technique also has drawbacks in that it is difficult to produce a laser beam output for monitoring and that the yield of high quality devices is very poor because one of the facets must be coated by five or more dielectric layers to provide the lower reflectivity.